1. Field of the Invention
This invention relates to a shift control device for an automatic transmission employing an electronic control, and more particularly to a shift control device which performs automatic gear-shifting of four speed or of three speed automatic transmissions including one with a mechanical drive with a torque converter lock-up clutch engaged.
2. Description of the Prior Art
Two types of shift control devices for automatic transmissions are known: one controls gear-shifting entirely by the hydraulic servo system and the other employs an electronic control to enable accurate gear-shifting at an exact vehicle speed. In shift control devices of the electronic type, as the number of gears increases the number of shift valves also increases, so that the number of electronically controlled solenoid valves increases thereby complicating the electric control circuit. For this reason, various shift control devices of the electronic type have been proposed in which the number of components such as shift valves is reduced. Conventional shift control devices, however, do not incorporate satisfactory countermeasures against erroneous operation due to sticking of valves and shocks produced when the shift lever is shifted from N-position to D-position. Furthermore, the amount of pressurized oil delivered from an oil pump to hydraulic circuits increases in proportion to the engine speed; therefore a small amount of oil pumped out when the engine is rotating at low speed is likely to result in the lack of oil supply. To reduce the amount of oil discharged and prevent the oil shortage in the hydraulic servo system, a large oil pump may be used or all the solenoid valves may be closed in the 1st gear when the engine is running at low speed. However, the former countermeasure has the disadvantage that a large portion of the engine output must be consumed for driving the large oil pump. The latter countermeasure has the disadvantage that if a fault or breakage of wire occurs while the vehicle is traveling at high speed, the solenoid valves will be deenergized and closed, causing a downshift to 1st gear and dangerous abrupt engine braking.